1. Field of the Invention
The present invention relates to an insulated gate semiconductor device which is employed as a switching element in a power converter such as an inverter, and more particularly, it relates to an improvement in a function for preventing an insulated gate element from breakdown in shorting of a load.
2. Description of the Background Art
An insulated gate element such as a MOS field effect transistor element (MOSFET) or an insulated gate bipolar transistor element (IGBT) has two current electrodes and a control electrode which is insulated from these electrodes, so that the value of a current flowing across the two current electrodes is adjusted by the value of a voltage which is applied across the control electrode and one of the current electrodes. The current is increased as the applied voltage is increased, while the same is cut off when the voltage is zero. An insulated gate semiconductor device comprising such an insulated gate element is employed as a switching element for a power converter such as an inverter for switching a current (main current) flowing to a load, for example. When the load is shorted in such a power converter, an excessive main current (short-circuit current) flows to the insulated gate element, which is finally broken if this state is left intact. In the insulated gate semiconductor device, therefore, a driving circuit for the insulated gate element, which is a circuit part for driving the insulated gate element, is provided with a short-circuit current cutoff function for preventing breakdown caused by such a short-circuit current.
FIG. 15 is a block diagram showing an exemplary conventional insulated gate semiconductor device having a short-circuit current cutoff function. A load (not shown) is connected to a collector C of an IGBT 1 serving as an insulated gate element, so that a collector current I.sub.C flowing from the collector C to an emitter E is supplied to the load as a main current. This collector current I.sub.C is controlled by the value of a voltage (gate voltage) across a gate G and the emitter E. A larger collector current I.sub.C flows as the gate voltage is increased. The gate voltage is regulated and supplied by a gate driving circuit 42.
This insulated gate semiconductor device is provided with a current transformer 43, which detects the collector current I.sub.C. The as-detected value of the collector current I.sub.C is compared with a prescribed reference value in a compare circuit 44. The compare circuit 44 transmits a prescribed signal to the gate driving circuit 42 when the collector current I.sub.C exceeds the reference value. In response to this signal, the gate driving circuit 42 outputs a prescribed gate voltage to the gate G, in order to cut off the IGBT 1. Thus, the excessive collector current I.sub.C resulting from shorting of the load is cut off to protect the IGBT I against breakdown.
Other exemplary conventional insulated gate semiconductor devices having short-circuit current cutoff functions are disclosed in Japanese Patent Laying-Open Gazettes Nos. 63-318781 (1988), 64-68005 (1989) and 2-309714 (1990). In each of the former two techniques, a second MOSFET is provided in parallel with a first MOSFET for controlling a main current to shunt the main current to the second MOSFET, while a transistor which enters an ON state when the shunt current exceeds a prescribed value is connected between gate and source electrodes of the first and second MOSFETs. When the main current flows in an amount exceeding the prescribed value due to shorting of a load etc., therefore, the transistor is turned on to reduce gate voltages of these MOSFETs, thereby limiting the main current below the prescribed value.
In the last one of the aforementioned conventional techniques, a thyristor is provided in place of the transistor provided in each of the former techniques. When a current which is shunted to a second MOSFET temporarily exceeds a prescribed value and a voltage exceeding a prescribed value is applied across a gate and a cathode of the thyristor, the thyristor so continuously conducts that gate voltages of two MOSFETs are reduced to values close to zero, whereby a main current is continuously cut off.
However, these conventional techniques have the following problems: It is particularly necessary to quickly cut off the short-circuit current when a high voltage is supplied to the load, and hence the gate driving circuit 42 and the compare circuit 44 must be driven at high speeds in the conventional technique shown in FIG. 15. When speeds of these circuits are increased, however, the circuits are liable to cause malfunctions by electrical noises and hence no stable operations can be obtained. Further, circuit loss is increased following such speed increase.
In the prior art of a system of limiting the gate voltages of the MOSFETs with the transistor, it is difficult to sufficiently reduce the gate voltages to values close to zero with the transistor, and hence the short-circuit current cannot be sufficiently cut off in shorting of the load. In the prior art employing a thyristor, on the other hand, the speed of response of the thyristor is so slow as compared with the transistor that a longer time is required for conduction of the thyristor after detection of an excessive main current as compared with the transistor. Therefore, an excessive short-circuit current flows in a constant period when the load is shorted, to break the MOSFETs during this period. Further, an excessive surge voltage is generated by inductance of the load since the current is cut off after a flow of such an excessive short-circuit current, also leading to breakdown of the MOSFETs.
In the prior art, further, the value of an ON-state voltage of the transistor, i.e., the value of a voltage signal which is supplied to the transistor for turning on the transistor, is varied with the temperature of the transistor. Thus, the value of the limited main current is disadvantageously fluctuated depending on the temperature. In the prior art, further, the transistor is erroneously turned on by an electrical noise which is superposed on the aforementioned voltage signal. An influence by such an electrical noise is increased as the speed of a switching operation for the MOSFETs is increased.